The present invention relates to a printing head using a plurality of printing wires.
A printing head comprising a plurality of printing wires, a plurality of armatures connected at their tips to the base ends of the printing wires, armature supporting means for rotatably supporting the plurality of armatures so that the tips of the armatures are movable in the longitudinal direction of the printing wires, and reciprocable drive means for driving the plurality of armatures to reciprocate the plurality of printing wires between a projected position and a drawn position in the longitudinal direction of the plurality of printing wires, has been already known by Japanese Patent Disclosure (Kokai) No. 60(1985)-124260.
In the printing head disclosed in Japanese Patent Disclosure (Kokai) No. 60(1985)-124260, the surfaces of the plurality of armatures are respectively coated with manganese phosphate films to prevent the rotatably supporting portions of the armatures from wearing on the armature supporting means. The manganese phosphate is a porous crystal which can hold lubricant, so that the lubricant is not splashed by the high speed operations of the armatures.
However, since the lubricant varies in its viscosity by temperature, the operating speeds of the armatures become irregular if the temperature of the lubricant largely alters due to the high speed operations of the armature and the temperature change in the circumferential environment of the printing head, thereby resulting in an instability in the impact forces of the printing wires and hence in a deterioration in the quality of printing.
The lubricant tends to absorb ink cake, printing sheet chips, and dust to cause the operations of the armatures to be disturbed.
Since the manganese phosphate films covered on the surfaces of the armatures are thin, the films are eventually worn to expose the surfaces of the armatures. At this time, the armatures fixed with the printing wires must be disposed, and this is uneconomical. Further, it is impossible to precisely control the thickness of the manganese phosphate film because the manganese phosphate is coated on the surfaces of the armatures by dipping the armatures in a mixture solution of primary manganese phosphate and phosphoric acid. This inhibits the armatures from being precisely controlled in size and hence prevents minimization the printing head size.